I know there are plenty of USB chargers out there for you to build. But here's one that doesn't use a voltage regulator or an IC chip to power it. The basic concept is to use 4 - AAA rechargeable batteries (1.2V a piece) to power the USB and a couple of solar cells to charge the batteries. If you've done the basic math you're probably wondering how 4.8V (4 x 1.2) is going to power a USB device which requires 5VDC. Well that's actually how I came up with the idea. I have another charger I made using a 9V battery and a 5V regulator. It will charge my iPod until the battery runs low. When I test the voltage output at the USB with a spent 9V, it will be spitting out 4.7V. So that's the cut off point. 4.8V is still within the tolerance for a USB device to charge. So I tried a 4-cell setup on my breadboard and it worked! I tested the voltage at the USB socket and (to my surprise) it was at 5.2V. I put my voltmeter on each battery and they were cranking out 1.3V at full charge. This is great! The tolerance of a USB happens to be just right for these four cells. So the solar part was sort of an afterthought for charging the batteries (they are rechargeable anyway).
If you'd like to try to make one yourself, here's what you need:
4 - AAA Battery Holder
USB Socket (female)
Solar Panel (3V output minimum, around 5V max)
Blocking Diode (not LED)
Some resistors (This is somewhat optional. You need them if you want your iPod/iPhone to recognize your charger. I'll explain exactly which ones to use later.)
DPDT Toggle Switch
Small Perf Board (Also optional)
Step 1: Prepare the Tin
I started with an easy step. Paint the tin. I just wanted to start with a blank canvas. You do too... trust me.